It is known that, for example in plants for transmission of high-voltage direct current, the properties of the power network connected to the convertor render difficult a feedback control of, for example, harmonics in the d.c. network. These difficulties may consist of the connected network exhibiting non-minimum phase properties but may also consist of the connected network, as a result of different operating configurations due to switchings, exhibiting different transmission properties at different times. The physical reason for the non-minimum phase properties of a power network can be explained by the fact that the events which take place along the power line in this connection must be regarded as electromagnetic waves which propagate along the power line and that these waves, at points where the impedance characteristic of the power line is changed, are reflected (see, e.g., Walter C. Johnson: Transmission Lines and Networks, McGraw-Hill International Editions 1984.) This entails multiple delay effects (echoes) in the response of the power network to control interferences since the attenuation of the travelling waves by the network itself is very low. It is known from textbooks in feedback control theory, for example Bernard Friedland: Control System Design, McGraw-Hill International Editions 1987, in particular pages 78, 144 and 188, note 4.7, and Richard C. Dorf: Modern Control Systems, Addison-Wesley Publishing Company, fourth edition 1986, in particular pages 262-264, that non-minimum phase properties for a transfer function entail zeros in the righthand half-plane of the complex s-plane and that in such cases it is not possible to achieve a stable feedback control with very short settling times.
In IEE Transactions on Industry Applications, Vol 24, No. 4, July/August 1988, pages 582-588, there is described a controller of repetitive type for the purpose of eliminating system deviations in the steady state while being influenced by cyclic disturbances. The introduction of a memory for storage of signal values in the controller makes it possible to achieve a good cancellation of a stationary cyclic disturbance also with a gain in the feedback loop which is low for reasons of stability. However, a low gain means that the contents of the memory is corrected relatively slowly during non-steady states.
The principal drawback in this case is that both delay effects and other non-minimum phase properties and other kinds of dynamic properties are not taken into consideration and compensated for. The correction of the memory contents with the system deviation delayed one cycle may render the controller unstable since in reality both implicit and explicit delay effects may occur in addition to other kinds of dynamic properties. The total delay in the system may amount to more than one cycle and even if the controlled system is stable, the attenuation of disturbances, repeated once every cycle, will not be good.
Complicated, varying or otherwise limiting dynamic properties for parts of power networks, caused by, for example, switchings or by non-minimum phase properties, thus make certain desirable control tasks very difficult or even impossible to solve.